1. Field of the Invention
The present invention relates to a circuit with variable capacitance and a method for operating a circuit with variable capacitance, and, in particular, the present invention relates to an electronic circuit with variable capacitance whose transition behaviour between a minimum and a maximum saturation value is variable.
2. Description of the Related Art
A conventional voltage controlled oscillator (VCO) in most cases has one or more characteristic tuning curves, wherein such a characteristic curve represents the relation between an applied tuning voltage Vtune and an oscillation frequency. An exemplary oscillator circuit for a conventional voltage controlled oscillator is illustrated in FIG. 4. The voltage controlled oscillator VCO includes a supply voltage terminal Vdd connected to a first terminal 404 of a first inductance L1 and a second terminal 406 of a second inductance L2 via a current source 402. The current source 402 is designed to impress the supply current Icore on the voltage controlled oscillator VCO. The first inductance L1 further includes a second terminal 408 electrically connected to a first electrode 410 of a first DC decoupling capacitor CDC1, a first terminal 412 of a first transistor T1 and a control terminal of a second transistor T2. In addition, a second terminal 416 of the second inductance L2 is electrically connected to a first electrode 417 of a second DC decoupling capacitor CDC2, a first terminal 418 of the second transistor T2 and a control terminal 420 of the first transistor T1. A second terminal 422 of the first transistor T1 and a second terminal 424 of the second transistor T2 are electrically connected to a ground potential terminal Vss. Furthermore, a second electrode 426 of the first DC decoupling capacitor CDC1 is electrically connected to a control terminal 428 of a first auxiliary transistor T3. A first terminal 430 and a second terminal 432 of the first auxiliary transistor T3 is electrically connected to a first terminal 434 and a second terminal 436 of a second auxiliary transistor T4 and the supply voltage terminal Vdd. Furthermore, a second electrode 438 of the second DC decoupling capacitor CDC2 is electrically connected to a control electrode 440 of the second auxiliary transistor T4. Furthermore, a control voltage terminal 442 for a control voltage Vtune is electrically connected to the control terminal 428 of the first auxiliary transistor T3 and the control terminal 440 of the second auxiliary transistor T4 via the decoupling resistors R1 and R2.
The voltage controlled oscillator VCO further comprises a first tap point A1 connected to the second terminal 408 of the first inductance L1, and a second tap point A2 connected to the second terminal 416 of the second inductance L2. Between the first tap point A1 and the second tap point A2, a voltage may be tapped off which may be output as differential output signal of the voltage controlled oscillator.
If a supply voltage is now applied between the supply voltage terminal Vdd and the ground potential terminal Vss, the oscillator circuit VCO illustrated in FIG. 4 settles such that either the first transistor T1 or the second transistor T2 is connected through. It may be assumed here that the serially connected capacitances CDC2, CDC1 and the auxiliary transistors T3 and T4 acting as voltage-dependent capacitances (varactors) may be regarded as a single total capacitance. If the first transistor T1 is connected through, the voltage controlled oscillator may thus be said to form an oscillating circuit between the supply voltage terminal Vdd and the ground potential terminal Vss with the second inductance L2 and the total capacitance, the frequency of the circuit being substantially adjustable by the total capacitance. The active area of the transistor thus has the width W and the length L. By applying the tuning voltage Vtune at the gate, the charge carrier situation within this active area as well as above and below changes. The result is a plate capacitor, so to speak, whose plate distance is changed by the control voltage (=tuning voltage) Vtune.
As a further interpretation of the circuit illustrated in FIG. 4, it is to be noted that the inductances L1 and L2 and the series connection of the right and left varactors (considering parasitic effects) may be regarded as only pertinent to an oscillating circuit. In such an oscillating circuit, there is then a high AC current whose magnitude is determined by its quality. The losses in this oscillating circuit are compensated in a phase-correct way by the smaller current Icore, for which the cross-coupled transistor pair T1 and T2 is responsible.
Furthermore, a current flow to the ground potential terminal Vss via the first transistor T1 is induced, also via the first inductance L1. If a potential at the first tap point A1 reaches a predetermined threshold by the current flowing across the first inductance L1, it is connected through via the control terminal 414 of the second transistor T2, resulting in an oscillating circuit between the supply voltage terminal Vdd and the ground potential terminal Vss via the first inductance L1 and the total capacitance. The oscillation frequency is again substantially determined by a capacitance value of the total capacitance. This capacitance value of the total capacitance may be adjusted by the tuning voltage (=adjusting voltage) which may be applied at the tuning voltage terminal 442. A capacitance value of the total capacitance may, in particular, be varied due to the fact that the capacitance of the auxiliary transistors T3 and T4 acting as varactors may be changed by the tuning voltage Vtune. The auxiliary transistors T3 and T4, which are preferably MOS transistors, are used such that the gate terminal acts as first electrode, the oxide between the gate terminal (=control terminal) and the substrate acts as dieletric and the (short-circuited) drain (=first terminal) and source terminals (=second terminal) act as second electrode of the varactor. Due to the fact that, using a variable voltage between thus connected auxiliary transistors T3 and T4, a channel width W/L of the channel forming between the gate terminal and the drain and source terminals is changeable, the capacitance value of the auxiliary transistors T3 and T4 acting as varactors may also be changed, resulting together in a change of the capacitance value of the total capacitance.
The more linear a relation between an applied tuning voltage and an oscillator frequency, the more favourable are its properties, for example when used in a phase locked loop (PLL). Particularly a voltage controlled oscillator VCO with constant inductance, as the inductances L1 and L2 illustrated in FIG. 4, requires a varactor to shift the resonant frequency according to the applied tuning voltage. As discussed above, particularly MOS transistor capacitances are used in voltage controlled oscillators produced in integrated circuit technology (such as in CMOS technology). For this, mainly the voltage-dependent capacitance between the gate and substrate (i.e. the drain and source terminals) is used.
However, such a varactor element consisting of the auxiliary transistors T3 and T4 illustrated in FIG. 4 have the disadvantage that the characteristic tuning curve generated therefrom is relatively short and thus steep in the transition area between the minimum and the maximum saturation value. Such a characteristic curve is shown in FIG. 5A, in which the VCO frequency is plotted as a function of an applied tuning voltage Vtune. This results in a sensitive transition area of the frequency/voltage characteristic, as illustrated in FIG. 5B. If a longer, i.e. flatter, tuning area with little curve inclination is to be covered, this may be done with many individual curves between which there must be constant switching. However, such a behaviour has the disadvantage that considerable hardware expenditures are necessary in the amplifier due to the necessary switching between the individual tuning curves, whereby it is not possible to produce such a voltage control oscillator at a low price.
Such problems concerning the steep capacitance behaviour of the capacitance as a function of the tuning voltage also occur in other fields of application, such as measuring technology where a maximally linear behaviour of the individual electronic devices is desirable over the entire dynamic range of measuring devices.